"It's fairly controversial just how active some groups of dinosaurs were and whether they actually had the capacity to be very active predators," said study coauthor Patrick O'Connor, a professor of gross and neuroanatomy at Ohio University in Athens. "We tackled the problem directly at the level of the lung."

"How much gas an organism exchanges is ultimately what dictates metabolism," he added. "In general terms, animals with higher metabolism were probably fairly active and energetic in their normal routinemuch like we think of mammals and birds today."

Metabolism and Lifestyle

Modern birds have complex respiratory systems, with two lungs and up to nine airs sacs. Sometimes found in hollow bones, the air sacs boost birds' respiratory volume and efficiency. As a result, birds can extract oxygen from the air much more efficiently than mammals can.

Nearly a century ago, scientists noticed that some dinosaur bones were "pneumatized," or possessed holes like those that house avian air sacs.

"But since nobody knew exactly which air sacs invaded what bones in birds, earlier speculation always lacked a solid basis," said Leon Claessens, a Harvard University paleontologist and study coauthor.

The two researchers examined the air sac systems of some 200 modern birds and identified specific patterns showing which air sacs are found in which bones.

The avian blueprint allowed the duo to make a striking comparison between bird anatomy and the fossilized remains of the meat-eating dinosaur species Majungatholus atopus.

"These features are spot on," O'Connor said. "You can take the [ Majungatholus] bone and hold it up to any number of bird bones and see virtually identical anatomy. It's compelling evidence."

The pair recently published their study in the science journal Nature.